1. Field of the Invention
This invention relates generally to determination of charge capacity of a battery pack and, more particularly, to a method for estimating the capacity of a lithium iron-phosphate battery pack after charging, where a property of lithium iron-phosphate battery resistance is used to enable determination of open circuit voltage during charging, and where characteristics of a voltage vs. state of charge curve for lithium iron-phosphate batteries are used to determine state of charge at two points on the voltage-charge curve, thus enabling actual charge capacity to be determined.
2. Discussion of the Related Art
Electric vehicles and gasoline-electric or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid-electric vehicles offer several desirable features, such as reducing or eliminating emissions and petroleum-based fuel consumption at the consumer level, and potentially lower operating costs. A key subsystem of electric and hybrid-electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle's cost. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life, and accurately informing the driver of the remaining vehicle range on battery power, are key considerations in the design and operation of electric and hybrid electric vehicles.
A typical electric vehicle battery pack includes two or more battery pack sections, with each section containing many individual battery cells as needed to provide the required voltage and capacity. In order to optimize the performance and durability of the battery pack, and to determine the amount of available charge capacity, it is important to monitor the state of charge of the battery pack. State of charge of a battery cell or a full battery pack is typically determined based on its open circuit voltage, using a known relationship which is defined in the form of a state of charge vs. open circuit voltage (SOC-OCV) curve. However, in some types of batteries, such as lithium iron-phosphate batteries, the extremely low slope characteristic of the SOC-OCV curve makes it very difficult to accurately determine state of charge based on open circuit voltage.
Furthermore, it is not feasible to simply measure charging and discharging current into and out of the lithium iron-phosphate battery pack in order to continuously determine state of charge, as small errors would accumulate over time to cause a large inaccuracy in such a measurement. Also, because a battery pack's capacity can fade over the life of the battery pack, an accurate understanding of driving range requires knowledge of both state of charge and battery pack capacity. A method is needed for accurately determining charge capacity of a lithium iron-phosphate battery pack, where the method does not suffer from the inaccuracies described above.